Generally, in wireless communications systems using frequency division duplex (FDD), knowledge of downlink (DL) channel information may be essential for efficient beamforming to one or more users. The transmission of information to two or more users is commonly referred to as spatial multiplexing.
DL channel information may be provided by a receiver (e.g., a mobile station, a user, a terminal, a User Equipment, and so on) to a transmitter (e.g., a NodeB, an enhanced NodeB, a base station, a base terminal station, a relay station, and so forth) over a feedback channel. The receiver may estimate or measure the DL channel and then feed the DL channel information back to the transmitter. The DL channel information may be feedback in its raw form, a quantized version (a codeword from a codebook known by both the receiver and the transmitter), an index to the quantized version (an index to the codeword from the codebook), or so on.
However, feeding back DL channel information over the feedback channel may be costly and unreliable, especially when the DL channel is changing rapidly in time and/or frequency, such as when the receiver is moving rapidly, when there is significant interference and noise present in an operating environment of the wireless communications system, or the UE is operating in a multipath environment with a large bandwidth.